In Dark mode, there isn't enough contrast between the active tab the the rest.
This new Safari is indeed faster. I use a benchmark program, the Speedometer 2.0 in browser bench.org. The score has gone up from 71 for the previous version to 81 for the current version. Out of curiosity, I did the benchmark for chrome as well. I got 87.
Apple has done a good job in improving Safari performance in this release. However, Safari used to out perform Chrome. Google has done an even better job as it appears.